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Can anyone identify this dinosaur?

So the other day I went to the Museum of Natural History and in the fossil hall I saw this beauty hanging from the ceiling:

Unfortunately though, I forgot to write down the specific name of this specimen and now that I am going through the photographs I made that day and storing them with species labels etc., this is the only one that I can't seem to find the name of by Googling or on the AMNH website (they only have a few of their fossils listed, not all). So I was hoping a dinosaur expert or AMNH connoisseur here can tell me the exact species name. I know it must be some kind of pliosaur, but which exactly?

FyreWulff is correct. Also, I'm almost certain that Ichthyosaurs were not technically dinosaurs.

Yeah, they were actually aquatic reptiles.

I'm not 100% that skeleton is an ichthyosaur, though. It probably is but the picture is taken at a weird angle and I can't really get a bead on what the skull looks like, but that's probably what it is.

Yeah, that and other kinds of genetic/cladistic reasons I think. IIRC Dimetrodons were actually really old - like, early Triassic/Mesozoic or maybe even in the very late Paleozoic - before true dinosaurs had became very prevalent. Dimetrodon is a big crazy reptile, but not a dinosaur, which is something specific and distinct from reptiles in general.

It's been a long time since I took that class, though, so somebody else can probably clarify much better than I can.

EDIT: Looks like Dimetrodon was actually a theropsid, or a mammal-like reptile and did in fact live in the Permian period. Neato.

Is the hip structure characteristic why things like Dimetrodon aren't considered dinosaurs either?

There are both "bird hipped" and "lizard hipped" dinosaurs. Ornithischian are the bird hipped, and Saurischian = lizard hipped. But, Dimetrodon wasn't a dinosaur, and it didn't stand up like dinosaurs...it's legs were more out to the side like a komodo dragon, or many of todays lizards.